| I've never built a QRD diffuser but I've made a lot of research - what you cannot afford the time to do now - so please take my advise with a bit of salt.
Diffusers are not meant to be used for absorbtion. They are meant to diffuse sound while keeping the acoustic energy. A very dead room isn't a good thing and for music halls, acoustic energy must be preserved and be heard everywhere as best as possible.
The principle of placing diffusers at least 3 wavelengths away from the listening position has to do with avoiding effects as similar as listening to a 3 way loudspeaker at a distance of 5". You get a lot of lobing that way and listening close to a diffuser tuned to a low frequency will have the same effect - actually it might make sound worse.
As many playback systems suffer from reduced high frequency playback level and high frequencies are more easily absorbed, it's best to tune an absorber to the 1kHz and above and enjoy the benefit it might provide in a large room. The back wall, close to the ceiling, is the usually recommended place to locate these.
QRD diffusers might cause "parasitic" absorbtion if they are not well crafted and seams are not airtight. These seams can behave like slat tuned absorbers and tune unpredictably. With a lot of research, calculations and probably trial and error, one might end up with something predictable and certain scenarios might even call for that.
But for small rooms (less than 30~60sqm) absorbption is actually what is needed and diffusers - although cool looking - might be an uncalled for overkill. |